Backwards-compatible ControllableClient implementation.

main.dart

mixin ClientControllerSupport {}

abstract class Client {
  void doSomething();
  
  static isControllable(Client client) {
    return client is ClientControllerSupport;
  }
}

class BaseClient extends Client {
  @override
  void doSomething() {
    print("Howdy!");
  }
}

class AuthClient implements Client {
  @override
  void doSomething() {
    print("Hello, world!");
  }
}

class NewAuthClient with ClientControllerSupport implements Client {
  @override
  void doSomething() {
    print("Hello, new world!");
  }
}

class OtherNewAuthClient extends BaseClient with ClientControllerSupport {}

void main() {
  
  AuthClient authClient = AuthClient();
  NewAuthClient newAuthClient = NewAuthClient();
  OtherNewAuthClient otherNewAuthClient = OtherNewAuthClient();
  
  print("authClient (e.g., legacy API)");
  authClient.doSomething();
  print(Client.isControllable(authClient));
  
  print("");
  
  print("newAuthClient (e.g., updated to support RequestController API)");
  newAuthClient.doSomething();
  print(Client.isControllable(newAuthClient));
  
  print("");
  
  print("otherNewAuthClient (e.g., extends BaseClient)");
  otherNewAuthClient.doSomething();
  print(Client.isControllable(otherNewAuthClient));
  
}